初步答案(EL 2.1,2009年5月)
如本Java论坛主题所述:
基本上,自动装箱会将Integer对象放入Map中。即:
map.put(new Integer(0), "myValue")
EL(表达式语言)将0评估为Long,因此在地图中寻找Long作为键。即它评估:
map.get(new Long(0))
由于aLong
永远不等于Integer
对象,因此不会在地图中找到该条目。
简而言之就是这样。
自2009年5月以来的更新(EL 2.2)
2009年12月,EL 2.2与JSP 2.2 / Java EE 6一起推出,与EL 2.1相比有一些区别。
似乎(“ EL表达式将整数解析为长”)是:
您可以在EL 2.2内部intValue
的Long
对象self上调用该方法:
<c:out value="${map[(1).intValue()]}"/>
这可能是一个很好的解决方法(下面在Tobias Liefke的答案中也提到了)
原始答案:
EL使用以下包装器:
Terms Description Type
null null value. -
123 int value. java.lang.Long
123.00 real value. java.lang.Double
"string" ou 'string' string. java.lang.String
true or false boolean. java.lang.Boolean
JSP页面演示了这一点:
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<%@ page import="java.util.*" %>
<h2> Server Info</h2>
Server info = <%= application.getServerInfo() %> <br>
Servlet engine version = <%= application.getMajorVersion() %>.<%= application.getMinorVersion() %><br>
Java version = <%= System.getProperty("java.vm.version") %><br>
<%
Map map = new LinkedHashMap();
map.put("2", "String(2)");
map.put(new Integer(2), "Integer(2)");
map.put(new Long(2), "Long(2)");
map.put(42, "AutoBoxedNumber");
pageContext.setAttribute("myMap", map);
Integer lifeInteger = new Integer(42);
Long lifeLong = new Long(42);
%>
<h3>Looking up map in JSTL - integer vs long </h3>
This page demonstrates how JSTL maps interact with different types used for keys in a map.
Specifically the issue relates to autoboxing by java using map.put(1, "MyValue") and attempting to display it as ${myMap[1]}
The map "myMap" consists of four entries with different keys: A String, an Integer, a Long and an entry put there by AutoBoxing Java 5 feature.
<table border="1">
<tr><th>Key</th><th>value</th><th>Key Class</th></tr>
<c:forEach var="entry" items="${myMap}" varStatus="status">
<tr>
<td>${entry.key}</td>
<td>${entry.value}</td>
<td>${entry.key.class}</td>
</tr>
</c:forEach>
</table>
<h4> Accessing the map</h4>
Evaluating: ${"${myMap['2']}"} = <c:out value="${myMap['2']}"/><br>
Evaluating: ${"${myMap[2]}"} = <c:out value="${myMap[2]}"/><br>
Evaluating: ${"${myMap[42]}"} = <c:out value="${myMap[42]}"/><br>
<p>
As you can see, the EL Expression for the literal number retrieves the value against the java.lang.Long entry in the map.
Attempting to access the entry created by autoboxing fails because a Long is never equal to an Integer
<p>
lifeInteger = <%= lifeInteger %><br/>
lifeLong = <%= lifeLong %><br/>
lifeInteger.equals(lifeLong) : <%= lifeInteger.equals(lifeLong) %> <br>